Allow Symbols in switch and maps

Allow Symbol literals (and const Symbol instances) as switch expression
and as key values in const Maps.

This implements the most recent Spec changes.

R=regis@google.com

Review URL: https://codereview.chromium.org//781203005

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42230 260f80e4-7a28-3924-810f-c04153c831b5
Fix scheduled_test's constraint on unittest.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//793493002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42231 260f80e4-7a28-3924-810f-c04153c831b5
Comment out the function causing CPU burn up.

TBR

R=brianwilkerson@google.com, danrubel@google.com
BUG=

Review URL: https://codereview.chromium.org//793433004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42232 260f80e4-7a28-3924-810f-c04153c831b5
Rollback r42232.

R=danrubel@google.com
BUG=

Review URL: https://codereview.chromium.org//785393003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42233 260f80e4-7a28-3924-810f-c04153c831b5